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<title>Problem 1683. -- [Usaco2005 Nov]City skyline -- 衡阳八中OJ离线版-2012-02-29</title><center><h2>1683: [Usaco2005 Nov]City skyline</h2><span class=green>Time Limit: </span>5 Sec&nbsp;&nbsp;<span class=green>Memory Limit: </span>64 MB<br><span class=green>Submit: </span>78&nbsp;&nbsp;<span class=green>Solved: </span>67<br>[<a href='submitpage.php?id=1683'>Submit</a>][<a href='problemstatus.php?id=1683'>Status</a>][<a href='bbs.php?id=1683'>Discuss</a>]</center><h2>Description</h2><div class=content>The best part of the day for Farmer John's cows is when the sun sets. They can see the skyline of the distant city. Bessie wonders how many buildings the city has. Write a program that assists the cows in calculating the minimum number of buildings in the city, given a profile of its skyline. 

The city in profile is quite dull architecturally, featuring only box-shaped buildings. The skyline of a city on the horizon is somewhere between 1 and W units wide (1 <= W <= 1,000,000) and described using N (1 <= N <= 50,000) successive x and y coordinates (1 <= x <= W, 0 <= y <= 500,000), defining at what point the skyline changes to a certain height. 

An example skyline could be: 
.......................... 
.....XX.........XXX....... 
.XXX.XX.......XXXXXXX..... 
XXXXXXXXXX....XXXXXXXXXXXX 
给出上面这样的图形，它将用数字化进行表示 


and would be encoded as (1,1), (2,2), (5,1), (6,3), (8,1), (11,0), (15,2), (17,3), (20,2), (22,1). 
(1,1)代表从第一列开始到第2-1列，有高度为1的"X" 
(2,2)代表从第二列开始到第5-1列，有高度为2的"X",这里的5-1是由上面输入的(5,1)中的第一个数字来决定的. 
(5,1)代表从第五列开始到第6-1列，有高度为1的"X" 
(6,3)代表从第六列开始到第8-1列，有高度为3的"X" 
.............................................. 
问最少用多少个矩形可以Cover所有的"X" 
This skyline requires a minimum of 6 buildings to form; below is one possible set of six buildings whose could create the skyline above: 

.......................... .......................... 
.....22.........333....... .....XX.........XXX....... 
.111.22.......XX333XX..... .XXX.XX.......5555555..... 
X111X22XXX....XX333XXXXXXX 4444444444....5555555XXXXX 

.......................... 
.....XX.........XXX....... 
.XXX.XX.......XXXXXXX..... 
XXXXXXXXXX....666666666666 </div><h2>Input</h2><div class=content>* Line 1: Two space separated integers: N and W

* Lines 2..N+1: Two space separated integers, the x and y coordinate
        of a point where the skyline changes. The x coordinates are
        presented in strictly increasing order, and the first x
        coordinate will always be 1.

</div><h2>Output</h2><div class=content>* Line 1: The minimum number of buildings to create the described
        skyline.

</div><h2>Sample Input</h2>
			<div class=content><span class=sampledata>10 26<br />
1 1<br />
2 2<br />
5 1<br />
6 3<br />
8 1<br />
11 0<br />
15 2<br />
17 3<br />
20 2<br />
22 1<br />
<br />
INPUT DETAILS:<br />
<br />
The case mentioned above<br />
</span></div><h2>Sample Output</h2>
			<div class=content><span class=sampledata>6</span></div><h2>HINT</h2>
			<div class=content><p></p></div><h2>Source</h2>
			<div class=content><p><a href='problemset.html?search=Silver'>Silver</a></p></div><center>[<a href='submitpage.php?id=1683'>Submit</a>][<a href='problemstatus.php?id=1683'>Status</a>][<a href='bbs.php?id=1683'>Discuss</a>]</center>﻿<br>

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